The concept of a negative index of refraction, originally proposed by Veselago in the 1960s, suggested the possibility of materials in which the permittivity and permeability could be made simultaneously negative. Veselago termed these left-handed media (LHM), because the vectors E, H, and k would form a left-handed triplet instead of a right-handed triplet, as is the case in conventional, right-handed media (RHM). In such a material the phase velocity and Poynting vector are antiparallel. Recently, novel three-dimensional (3D) electromagnetic materials have successfully demonstrated negative refraction of two-dimensional (2D) electromagnetic waves by synthesizing a negative refractive index. These artificial dielectrics (metamaterials) consist of loosely coupled unit cells composed of thin wire strips and split-ring resonators to synthesize negative permittivity and permeability, respectively. In these metamaterials, the choice of operating frequency is restricted to the region of resonance, which results in a highly dispersive, narrowband behaviour with strong associated absorption losses.
Other structures with magnetic properties to refract electromagnetic radiation at negative angles have also been considered. For example, Inteniational PCT Application Publication No. WO 00/41270 discloses a structure that exhibits magnetic properties when it receives incident electromagnetic radiation. The structure includes an array of capacitive elements, each of which is smaller, and preferably much smaller, than the wavelength of the incident electromagnetic radiation. Each capacitive element has a low-resistance conducting path associated with it and is such that a magnetic component of the incident electromagnetic radiation induces an electrical current to flow around a path and through the associated capacitive element. The creation of the internal magnetic fields generated by the flow of the induced electrical current gives rise to the structure's magnetic properties.
International PCT Application Publication No. WO 02/03500 discloses a microstructured magnetic material having a magnetic permeability of negative value but unity magnitude over a particular radio frequency range.
Furthermore, in all the previous structures described, the unit cells are not physically connected. This further restricts their useful operating bandwidths.
U.S. Patent Application Publication No. US-2004-0066251-A1 discloses improved left-handed metamaterials exhibiting negative refractive indices. The left-handed metamaterials incorporate transmission lines loaded with discrete components. Fabricating such a metamaterial is more costly and difficult than fabricating a metamaterial with unloaded i.e. continuous transmission lines. Moreover due to the discrete embedded elements it is challenging to extend their operating frequencies well into the microwave or millimetre-wave spectra.
Although metamaterials exhibiting negative refractive indices exist, improved metamaterials that are easier and less costly to manufacture are desired. It is therefore an object of the present invention to provide novel isotropic “left-handed” and anisotropic “hyperbolic” negative-refraction metamaterials for controlling and guiding electromagnetic radiation.